1. Field of the Invention
The invention relates generally to display systems with electromagnetically deflected cathode ray tube random stroke and periodic raster displays, and more particularly to a system of the type described having a bi-directional raster scan with reduced power consumption when operating in a patch raster mode.
2. Description of the Prior Art
A primary concern in electromagnetically deflected cathode ray tube display systems is the significant increase in power consumption with larger, faster and brighter displays. The power efficiency of deflection systems that display both raster and stroke writing is relatively low due to the high yoke driving voltages required to assure adequate writing speed and linear operation to supply accurately controlled currents to the deflection yokes.
An additional area of concern is the slew rate of the deflection amplifiers. The slew rate design criteria (which also determine peak power requirements) are determined by the display content and format requirements. By utilizing a bi-directional raster which writes alternate lines of raster in opposite directions, the power required for fly-back is substantially reduced and additional information may be displayed during the retrace period without increasing the supply of voltages.
In U.S. Pat. No. 4,678,970 filed May 1, 1986 by Winston W. Goldman for Power as Required Beam Deflection System for CRT Displays with Raster Supply Switching, which patent is assigned to the assignee of the present invention, a system was described for reducing power during the raster display as well as during the stroke display. The invention provided for switching the power supplied to the yoke driver during the raster deflection period to the minimum power level required by the deflection wave form, as well as switching supply levels between the raster and stroke deflection periods. An accurate magnetic deflection system requires linear power amplifiers with feedback loops to provide position accuracy and long-term stability. Because the amplifiers operate in the linear mode, they dissipate large amounts of power. While power switching of the amplifier power supplies was provided by the aforementioned patent, it did not provide for patch raster capability. Patch raster allows the deflection drive to produce a visible raster segment anywhere on the CRT display screen without requiring scanning through the screen center. This asymmetrical scan saves raster scanning time by allowing the deflection system to scan only the area of interest. The prior art power switching systems are not capable of this patch raster operation and dissipate significantly great amounts of power. U.S. Pat. No. 4,678,970 used a power switching system that returned excess energy to the stroke power supplies. If patch raster was attempted, it was found that the added energy would cause an over-voltage condition in the stroke power supplies, resulting in damage to the deflection amplifier or the power supply itself. It is therefore desirable to provide an electromagnetic display deflection system which allows patch raster operation without adversely affecting the power supplies or deflection amplifier and which reduces energy consumption.
The present invention differs from the prior art by returning energy to the same supply from which energy was drawn during the previous raster scan line. The system returns energy at the raster scan line rate to the more heavily loaded of the raster supplies. This technique requires smaller decoupling capacitors and causes no power supply malfunction while allowing patch raster operation. Further, it results in saving a significant amount of power that would otherwise be dissipated in the deflection amplifier power stage.